1. Field of the Invention
This invention relates to a reinforcement for composite materials, metallic composite materials, and ceramic composite materials. More particularly, it relates to a reinforcement for composite materials which is useful as a reinforcing material combined with high-temperature-melting titanium alloys or various ceramics to provide metal matrix composites (hereinafter abbreviated as MMC) or ceramic matrix composites (hereinafter abbreviated as CMC), metallic composite materials excellent in heat resistance, corrosion resistance and mechanical properties, and ceramic composite materials excellent in heat resistance, corrosion resistance and mechanical properties.
2. Description of the Prior Art
Various materials having excellent heat resistance and mechanical properties have hitherto been developed for use in the field of aerospace and aircraft. Typical materials proposed include metal matrix composites (MMC) and ceramic matrix composites (CMC).
Composite (1) described below can be mentioned as an example of MMC.
(1) Composite formed by using an .alpha.+.beta. type titanium alloy, e.g., Ti--6Al--4V, or .alpha.+.beta. type titanium alloy, e.g., Ti--15V--3Cr--3Sn--3Al, as a matrix and SiC/C composite fiber as a reinforcement. The above-mentioned titanium alloys are superior to steel, superalloys or heat-resistant eutectic alloys in specific strength, specific modulus and corrosion resistance and also have a higher working temperature limit (e.g., 723K) than aluminum alloys. The SiC/C composite fiber as referred to above comprises a carbon fiber core having deposited thereon SiC by chemical vapor deposition (CVD). For example, the SiC/C composite fiber produced by Textron Co. is most widely used.
It is reported that composite (1) exhibits high performance owing to the use of SiC/C composite fiber which exerts its full performance as a reinforcement when combined with the above-mentioned titanium alloy (see The Journal of the Japan Society of Composite Materials, Vol. 17, No. 1, p. 25 in pp. 24-31 (1991)).
In case where any other heat-resistant fiber, for example, carbon fiber (e.g., PAN-based or pitch-based carbon fiber), Al.sub.2 O.sub.3 fiber (e.g., products produced by E. I. du Pont de Nemours & Co., Inc., 3M Co., and Sumitomo Chemical Co., Ltd.), SiC fiber (e.g., "Nicalon", produced by Nippon Carbon Co., Ltd.), Si--Ti--C--O fiber (e.g., "Tyranno Fiber", produced by Ube Industries, Ltd.), Si--C--N-based fiber (e.g., "HPZ Fiber", produced by Dow Corning Corp.) or Si.sub.3 N.sub.4 fiber (e.g., a product produced by Tonen K.K.) is used as a reinforcement in place of SiC/C composite fiber, it undergoes considerable reaction with the above-described titanium alloy upon composite molding to a degree varying according to the kind of the fiber. As a result, no reinforcing effect is produced at all or, even if the degree of deterioration by the reaction may be small, substantial reinforcing effects cannot be obtained, so that the resulting composite material fails to display sufficient performance for use in the field of aerospace and aircraft industries.
However, composite (1) involves drawbacks (a) and (b) as follows.
(a) Because the carbon fiber core and the surrounding SiC in the reinforcement are greatly different in coefficient of thermal expansion, the SiC/C fiber has considerably poor thermal fatigue characteristics (resistance to fatigue of repeated alternation of low temperature and high temperature). Therefore, the SiC/C fiber has insufficient reliability for use as a reinforcement in high-temperature structural members.
(b) Because the SiC/C fiber is prepared by CVD, it is so expensive (at least .Yen.1,000,000/kg) that the application is limited to specific military uses where no consideration of price is required. In the aircraft industry, the cost is a today's problems. Seeing that an acceptable price of reinforcements, the cost of a reinforcements is .Yen.200,000/kg to .Yen.300,000/kg at the highest. Thus the SiC/C fiber is impractical.
MMC also includes composite (2) formed by using, as a matrix, an intermetallic compound which is more heat resistant than the titanium alloys used in (1) above, such as Ti.sub.3 Al, TiAl or Nb.sub.3 Al, and, as a reinforcement, SiC/C fiber, taking deterioration by reaction on composite molding into consideration as in (1) above.
Under the present situation, however, the SiC/C fiber in composite (2) still involves the problem of deterioration by reaction at the time of preparing a composite, failing to manifest its performance to the degree displayed in composite (1).
MMC so far proposed further includes composites (3) and (4) described below.
(3) Composite formed by using an aluminum alloy (e.g., A6061, A2024 or A1070) as a matrix and SiC/C fiber (e.g., a product of Textron Co.) (see The Journal of the Japan Society of Composite Materials, Vol. 17, No. 1, p. 25 (1991)), Al.sub.2 O.sub.3 fiber (e.g., a product of Sumitomo Chemical Co., Ltd.) (see Kobunshi Gakkai (ed.), Kobunshi Shinsozai Binran, p. 463, Maruzen (1989)), carbon fiber (e.g., Tetsu no Ko, Vol. 75, No. 9, pp. 41-48 (1989)) or SiC fiber (e.g., "Nicalon", produced by Nippon Carbon Co., Ltd.) (see Kobunshi Gakkai (ed.), Kobunshi Shinsozai Binran, p. 472, Maruzen (1989)) as a reinforcement.
However, composite (3) using SiC/C fiber is disadvantageous in that the reinforcing fiber is expensive and considerably inferior in thermal fatigue characteristics, although high strength in low temperature is obtained. The composite using carbon fiber is disadvantageous in that it does not withstand an oxidizing atmosphere at 200.degree. C. or higher for a long time and has insufficient corrosion resistance due to the conductivity of the fiber. Further, Al.sub.2 O.sub.3 fiber or SiC fiber reacts with an aluminum alloy, e.g., A6061 or A2024, which is the most practical as a matrix, upon MMC molding and thus suffers from deterioration, failing to exhibit sufficient reinforcing effects. As a result, the mechanical properties of the composite using these fibers are far below the theoretical values (ROM values).
(4) Composite formed by using a magnesium alloy (e.g., ZE41 or AZ91) as a matrix and SiC/C fiber as a reinforcement (see The Journal of the Japan Society of Composite Materials, Vol. 17, No. 1, p. 26 (1991)).
Similarly to composite (3), composite (4) is notably inferior in thermal fatigue characteristics while exhibiting high performance in low temperatures.
MMC composed of a magnesium alloy and inorganic fiber other than SiC/C fiber, such as Al.sub.2 O.sub.3 fiber or SiC fiber, has also been proposed. However, since a considerable reaction occurs between the fiber and the magnesium alloy in the preparation of MMC to deteriorate the fiber, the fiber cannot exert its full performance.
CMC which has been proposed to date include composites (5) to (8) shown below.
Si--Ti--C--O fiber (e.g., "Tyranno Fiber", produced by Ube Industries, Ltd.) and SiC fiber (e.g., "Nicalon", produced by Nippon Carbon Co., Ltd.) are most widely employed as a reinforcement in conventional CMC because of their heat resistance, oxidation resistance, and performance stability (being mass-produced and commercially available).
(5) Composite formed by using Nicalon as a reinforcement and Li.sub.2 O.sub.3.Al.sub.2 O.sub.3.SiO.sub.2 (LAS-I, glass ceramics) as a matrix (see J. Mater. Sci., Vol. 17, pp. 2371-2383).
(6) Composite formed by using Nicalon 2D fabric (plane weave) as a reinforcement and SiC prepared by chemical vapor impregnation (CVI) (see Am. Ceram. Soc. Bull., Vol. 65, No. 2, pp. 336-338 (1986)).
(7) Composite formed by using Tyranno 3D fabric as a reinforcement and SiC prepared by CVI as a matrix (see The Japan Society of Mechanical Engineers (ed.), The 70th JSME Spring Annal Meeting (I), 1993--3.31 to 4.2, pp. 163-166).
(8) Composite formed by using Tyranno fiber as a reinforcement and glass ceramics (i.e., the above-mentioned LAS, BaO--MgO--Al.sub.2 O.sub.3.SiO.sub.2 or CaO--Al.sub.2 O.sub.3.SiO.sub.2) as a matrix.
Composites (5) to (8) above described have the following characteristics.
Composite (5) as a unidirectionally (0.degree.) reinforced composite has a maximum bending strength of 900 MPa and an extremely high K.sub.lC (fracture toughness value) in the range of from 17 to 25 MPa.sqroot.m (the K.sub.lC of general SiC moldings is 3 to 5 MPa.sqroot.m and that of general Si.sub.3 N.sub.4 moldings is 7 to 9 MPa.sqroot.m).
Composite (6) has an extremely high K.sub.lC of about 27 MPa.sqroot.m.
The K.sub.lC of composite (7) is still higher than that of composite (6) and is estimated to be 50 MPa m or higher from the stress strain curve in tensile strength measurement. The tensile strength of composite (7) is 400 MPa, which is double that of composite (6) (200 MPa). Taking the three-dimensional structure (3D) and the fiber volume fraction (Vf) of 40 vol % into account, the above K.sub.lC value is approximate to a theoretical value (i.e., fiber strength 3000 MPa/3.times.0.4=400 MPa).
Composite (8) exhibits markedly improved strength as compared with composite (1).
Nevertheless, composites (5) to (8) have the following disadvantages.
In composite (5), since Nicalon and LAS has slightly reacted in the preparation of CMC, the strength is lower than the theoretical value (ROM: 1400 to 1500 MPa in this case) obtained from the mechanical property of the fiber and fiber volume fraction (Vf) in CMC.
In composite (6), the fiber is embrittled due to too strong bonding between Nicalon and SiC so that the composite has a bending strength of 300 to 400 MPa and a tensile strength of 200 MPa, which are fairly lower than the theoretical values (max. 1200 MPa).
The oxidation-resistant temperature of composites (5) and (6) is from 1000 to 1200.degree. C. and that of composites (7) and (8) is from 1200 to 1300.degree. C. Composites (5) to (8) are therefore unsatisfactory in terms of heat resistance and oxidation resistance demanded as an advanced material withstanding the extreme conditions in the future aerospace and aircraft industry.
Development of CMC using a matrix other than the glass ceramics or SiC formed by CVI with having a high porosity as used in composites (5) to (8), i.e., Si.sub.3 N.sub.4 excellent in corrosion resistance or highly heat-resistant SiC formed by ordinary sintering, has been awaited. However, production of CMC using Si.sub.3 N.sub.4 or SiC as a matrix needs a high temperature (e.g., 1600 to 2100.degree. C.) in sintering, and that causes remarkable deterioration of inorganic fiber due to the induced reaction. For example, Nicalon used as a reinforcement loses its fiber shape completely during molding in the production of the above-described CMC. The same problem also occurs in the case of using SiC/C composite fiber, Al.sub.2 O.sub.3 fiber, Si.sub.3 N.sub.4 fiber or Si--C--N fiber as a reinforcement. In using Tirano fiber as a reinforcement, the fiber shape can be retained to show reinforcing effects to some extent, but the strength reached is far from the CMC theoretical value (ROM value).
Hence, the conventionally proposed MMC and CMC do not satisfy the conditions of heat resistance, corrosion resistance, mechanical properties, and the like which are required in the field of aerospace aircraft industries because the fiber reinforcement reacts with the matrix in the production of these composite materials and also the composites themselves are inferior in thermal fatigue characteristics, heat resistance, and oxidation resistance. It has therefore been demanded to develop a reinforcement which is free from the above-mentioned disadvantages and sufficiently exhibits heat resistance, mechanical properties, or the like performance.
Further, the conventional reinforcement, when used at a low Vf, tends to be hard to disperse in a matrix uniformly, causing dispersion unevenness, to provide a composite with uneven strength. Such being the case, the reinforcement cannot help but be used in an amount over the specification, resulting in increased production cost. If the Vf is high, cases are sometimes met with, in which individual fibers come into contact with each other, failing to provide a composite having desired properties, such as mechanical strength.